During short-time arc welding, an element is welded onto a component. In this case, an arc is formed between the element and the component, which melts the end surfaces. The element and the component are then moved towards one another, so that the melts mix. The arc is short-circuited, and the entire melt solidifies.
Normally, the arc is drawn (welding with drawn arc). In this case, the element is first of all placed on the component. A pilot current is then switched on, and the element is raised to a desired height above the component, with an arc being drawn. The welding current is not switched on until this point.
In order to achieve constantly good welding results, it is important, inter alia, to know the relative position between the element and the component, in particular to raise the element to the correct height before the welding current is switched on. To this end, the relative position is generally measured before each welding process, in particular in the form of a null position being determined.
This is particularly important for robot-based systems. Modern robots are admittedly generally able to position themselves comparatively precisely. However, particularly owing to the large moving masses, high precision cannot be achieved at the same time as a very fast dynamic response. The robots can normally move in three co-ordinates. In the simplest case, a robot is a linear guide (slide) which is driven in an automated manner and on which a welding head is mounted.
Stud welding systems are used in particular in the motor vehicle industry where they are used, in particular, to weld elements such as bolts or studs with or without a thread, eyelet, nuts etc. to the sheet-metal bodywork. These elements are then used as holding anchors in order, for example, to fix internal linings.
The production speed is a major factor in the motor vehicle industry. Hundreds of the elements must be welded automatically by means of robots at different positions within a few minutes. In consequence, the robots have to have a short-time dynamic response.
It is thus known for a welding head base supporting a slide to be fitted to the arm of a robot. The slide can be moved highly dynamically with high precision, normally by means of a pneumatic or hydraulic system. The actual welding head is mounted on the slide and itself has a linear movement apparatus for moving the element.
In order to determine the relative position between the element and the component, it is known for a so-called supporting foot to be attached to the welding head (for example from “Neue TUCKER-Technologie. Bolzenschweiβen mit System”, Emhart Tucker, September 1999). The supporting foot is aligned approximately parallel to the holding apparatus of the welding head. In an initial position, the element which is held in the holding apparatus projects somewhat beyond the supporting foot.
In order to determine the relative position, the welding head is moved towards the component. In the process, the element first of all makes contact with the component. The welding head is fed further forwards until the supporting foot makes contact with the component. In the process, the holding apparatus is generally offset relative to the welding head, against an elastic biasing force. The interlocking contact between the supporting foot and the welding head, together with a suitable measurement system in consequence allow the relative position between the element and the component to be determined.
As an alternative to this, so-called measurement systems without supporting feet are also known for determining the relative position between the element and the component. For example, U.S. Pat. No. 5,252,802 discloses a bolt welding apparatus with a housing which is in the form of a handheld pistol. A position motor first of all moves the housing to a position in which a bolt is arranged in the vicinity of a component. A linear motor is provided in the housing, in order to move a linear-movement shaft, which carries the stud, axially. A movement measurement system is provided in order to control the linear motor. To determine the relative position between the stud and the workpiece, the linear motor is driven in order to move the stud towards the workpiece at a predetermined speed. As soon as the stud touches the workpiece, an electrical contact is closed. Furthermore, it is known from WO 96/11767 for the stud holder to be elastically biased in the direction of the workpiece, and to be moved axially against the bias force by means of a linear motor.
WO 96/05015 also discloses a stud welding apparatus without a supporting foot. However, stud welding without a supporting foot has the disadvantage that, particularly if the components are thin metal sheets, it is not always possible to exactly maintain an exact position relationship between this component and the element or bolt. This is due to the fact that thin sheet metal is frequently bent somewhat when the bolt makes contact with it. As soon as the bolt is lifted off the metal sheet again during the process of raising it to draw an arc, the metal sheet moves back, and the exact position relationship is lost.
Essentially, this problem does not exist in stud welding with a supporting foot. This is because the supporting foot makes contact with the component or metal sheet throughout the entire welding process. In consequence, this mechanical coupling always makes it possible to maintain an exact position relationship.
However, stud welding with a supporting foot has the disadvantage that the supporting foot, which is necessarily arranged close to the stud, increases the amount of space required for stud welding. Furthermore, spot welds must generally be arranged where the component is planar. In the region of steps or depressions, it is frequently necessary to rotate the arm of the robot so that the supporting foot does not impede the movement of the welding head. This means that the supply lines to the welding head are subject to high stress levels resulting from torsion, compression etc.